MANILA, Sept. 29 (Xinhua) -- The U.S. Embassy in Manila on Tuesday said that the two American soldiers who were killed in a blast in Mindanao Tuesday were not involved in combat operations.

In a statement, the embassy said the incident occurred at 8:45 a.m. while the servicemen were conducting a resupply mission for a school construction project in Jolo.

An investigation is underway to determine the details of the incident, the embassy said.

Apart from the two U.S. service members, a Philippine Marine was also killed while two other Armed Forces of the Philippines members were seriously wounded when their vehicle struck an improvised explosive device.

"The U.S. Embassy in Manila and the U.S. members of Joint Special Operations Task Force-Philippines express deepest sympathy and condolences to the families and loved ones of the killed and wounded Philippine and U.S. soldiers," the embassy statement said.

"They lost their lives serving others and we will always be grateful for their contributions to improve the quality of life on Jolo," said U.S. Ambassador Kristie Kenney.

The Department of Foreign Affairs backed the embassy's statement, saying the American troops died while doing civic and humanitarian work.

"It is with sadness that we learned of the death of a Philippine Marine and two US servicemen. What they were doing in Sulu--assisting Philippine military and local government officials in undertaking civic engineering projects, protecting families and securing peace---are important to the Filipino people," a department statement said. "Our prayers and sympathies go out to their families and loved ones."

No group has claimed responsibility for the attack, the military said.

The presence of American troops in Mindanao since 2002 has fanned speculations that they are involved in combat operations against Al-Qaida-linked terrorists in Mindanao like the Abu Sayyaf, which is on the U.S. government's list of the foreign terrorist organizations.